I propose to determine the essential domains of the Trk receptor tyrosine kinase (RTK) used for neurotrophic signaling in primary neurons. The Trk gene family encodes receptors for the NGF related neurotrophins which promote neuronal survival and maintain neuronal function. Like other RTKs', ligand binding stimulates phosphorylation at internal tyrosine residues; leading to a downstream cascade of signaling. To date, Trk signal transduction studies has been done on rat PC12 pheochromocytoma cells and mouse 3T3 fibroblast cells. NGF stimulation of trk leads to neurite outgrowth in PC12 cells while it induces proliferation in 3T3 cells, indicating that critical elements of the trk signaling pathway can vary. For this reason, we consider it important to study Trk receptors in a context that closely approximates the in vivo condition. While primary embryonic neurons are ideal substrates for the proposed studies, biochemical studies are not possible with these limited cell numbers. Instead I have opted for a molecular approach, aiming to introduce recombinant adenoviruses into post-mitotic neurons. The potential complication of having endogenous Trk receptors in neuronal cells will be avoided by culturing neurons from knock out mice. Our laboratory has recently developed mouse models harboring mutations that inactivate the Trk receptors. Using the Trk null mice, we now can culture Trk negative neuronal cells. I propose to employ a replication-defective adenovirus as a vehicle to introduce mutated Trk receptors into primary neurons in culture. The working hypothesis is that specific intracellular mutations in the Trk receptor will impede normal signaling and result in altered neuronal phenotypes.